TOPIC:

There are 3 ports. Have a read through this thread supplied by our Carburetor Guru Scotch. He should have been on the design team for the BSW32 Mikuni's. Maybe they wouldn't be such a bitch to work on had he had input on the design.

Off Idle Flat Spot Issues

You also mentioned that you borrowed a "Scotch's Cleaning Tool " from a buddy and cleaned the carbs. Did you follow the instruction sheets that come with the tool?

 

my theory is that gasoline actually lubricates the slides
The slides should  never see fuel !
I’m going to raise the fuel level next and see if that changes anything.
Don't !
  surely the vacuum "is what it is" at idle?
Until you sync. If you find, when you start, that one is at 12" and two are below...for example, 7"..... TRUST ME; You'll NEVER get the two lower ones to achieve 12". Lower the highest first and the two lower ones will rise. Balance equally from there. And: regardless of whether you use gauges or "sticks"......... DAMPEN them !!    If your Indicators are bouncing 2" per reading then you might as well just tune by ear.　
There's lots of reports of guys trying to sync their carbs and the intake vacuum at idle reported down at 7-8 in mercury. Scotch can chime in here. He just rebuilt his engine and I think his intake vacuum at idle jumped up to 10 In HG ??
Correct !　
If the initial ignition timing is off or the valve timing is off, or the compression is off, the engine won't run at idle.　
That's a fact !  Advance and Retard the ignition timing slightly. You may very well be surprised at this relationship with the vacuum readings, idle RPM and the smothness of the idle.

Another simple elimination of possibilities is:  Pull all of your Pilot-Needles and look at the Tips, with some decent magnification.  Any   deformation: mushrooming and/or the slightest bend.... Replace them.   

Thanks Scotch, I’ve just finished stripping and  cleaning with your carb tool and rebuilding the carbs yet again. I was quite surprised how many little bits of debris came out and just how discoloured the thinners was even though this is the third time I’ve cleaned them. I must admit this time I did them exactly as your instructions where as before I might have cut a few corners lol. I’m not refitting the carbs till tomorrow so while they’re on the bench I’ll have a look at the pilot screws through my jewellers eyeglass. 
Also while they where on the bench I’ve removed the enrichment diaphragms and put the cover back on with a gasket. 
thanks once again for your input and patience. I’ll report back tomorrow when I fire her up again. Fingers crossed that I’ve sorted it. 

Well finally she’s running properly, bolted the carbs back on and after a bit of balancing she runs as sweet as a nut. The tickover is a bit fast and the adjuster is fully backed off so I think I need to redo the balancing. 
Thanks to everyone for their input, hopefully i can just enjoy  riding  the beast now. 

Glad to hear it all worked out.

I've said all along, Scotch's Cleaning Tool is a well designed piece. There's no trick to using the tool but following the directions provided with the tool is the difference between doing it once and getting it right or fighting Pilot Circuit blockages tooth and nail as this thread has proved.

Genius is hard to follow without a road map.